Step 1. Step up long addition
1 . 4 2 0
+ 2 . 1 5 7
_________
Step 2. Calculate 0 + 7, which is 7.
1 . 4 2 0
+ 2 . 1 5 7
_________
7
Step 3. Calculate 2 + 5, which is 7.
1 . 4 2 0
+ 2 . 1 5 7
_________
7 7
Step 4. Calculate 4 + 1, which is 5.
1 . 4 2 0
+ 2 . 1 5 7
_________
5 7 7
Step 5. Calculate 1 + 2, which is 3.
1 . 4 2 0
+ 2 . 1 5 7
_________
3 . 5 7 7
Step 6. Therefore, 1.42 + 2.157 = 3.577
3.577
Done!